HARARE, Zimbabwe (AP) — President Robert Mugabe ceded some power in Zimbabwe for the first time in 28 years, signing a power-sharing deal Monday with opposition leader Morgan Tsvangirai amid questions on how the fierce enemies will work together to fix the collapsing economy. Thousands of supporters of the rival parties threw stones at each other as the ceremony got under way and several hundred broke through the gates of the convention center where it took place. Police fired warning shots and set dogs on the crowd, which calmed and cheered as their leaders left after the signing. Mugabe, Tsvangirai and Arthur Mutambara, leader of a faction that broke away from Tsvangirai's party, all pledged to make the deal work. But long-simmering and bitter differences as well as the nation's economic collapse — inflation is officially running at 11 million percent — have put the deal under intense pressure. It has already been criticized privately by some opposition leaders, who are unhappy that it leaves Mugabe as president and head of the government. They fear Mugabe will exploit that, especially by playing on tensions between the two opposition groups. Nine African leaders including mediator President Thabo Mbeki of South Africa witnessed the signing in a show of commitment to the deal, which the African Union is underwriting. Tanzania's President Jakaya Kikwete, chairman of the African Union, voiced the concern on many minds: "Will it hold or will it not? That is the question," he said. Aid agencies welcomed the deal as a hopeful sign they will be able to step up food deliveries to millions of people facing hunger. "The food situation in Zimbabwe has reached crisis point," said Matthew Cochrane of the international Red Cross. "There are already more than 2 million people who don't have food, and that number is going to rise to 5 million, which is about half the country's population, by the end of the year." Mugabe's government restricted the work of aid agencies in June, accusing them of siding with the opposition before a presidential runoff. The ban was lifted last month, but aid agencies say it takes time to gear up. Mugabe, 84, has been in power since independence in 1980 and went from being praised as a liberator who freed the former British colony from minority white rule to being vilified as an autocrat. He and Tsvangirai, 56, have been enemies for a decade, and Tsvangirai has been jailed, beaten, tortured and tried for treason — charges that were dismissed in court. Under a complicated arrangement, Tsvangirai is the prime minister with executive powers to chair a new council of ministers responsible for forming government policies. He is deputy chairman of a Cabinet of ministers that Mugabe will head. The agreement provides for 31 ministers — down from 50 — 15 nominated by Mugabe's party, 13 by Tsvangirai and three by Mutambara. The parties began negotiating Monday which ministries they would hold and an announcement was expected later this week. Opposition leaders want the Home Affairs Ministry that would give them charge of the police who have terrorized them and their supporters this year, and Mugabe would retain the Defense Ministry. Tsvangirai saluted members of parliament for their willingness to work across party lines. "If you were my enemy yesterday, today we are bound by the same patriotic duty," he said, calling on legislators to be "driven by the hope of a new, better, brighter country" and the "hope of a new beginning." Tsvangirai said his first priority would be getting food to hungry Zimbabweans. Mugabe, in a speech that was often combative, again accused Britain and the U.S. of wanting regime change in Zimbabwe and sneered at the democratic process. "This thing called democracy is a problem. It's a difficult proposition because always the opposition will want much more than what it deserves," he said. A new constitution will be drafted, with input from civic groups. Consultations will start within a month and a referendum on the constitution will be held within two years. Western countries say they will withhold the aid vital to help the country rebuild until it is clear Mugabe is committed. EU foreign ministers welcomed the deal but said Mugabe must prove he is willing to restore democratic rule before sanctions can be lifted. The EU sanctions impose a travel ban and assets freeze on Mugabe and 171 people and four companies sustaining his old government. The bloc also has frozen aid projects in Zimbabwe and imposed an arms embargo. British Foreign Secretary David Miliband said restoring ties and scrapping sanctions depended on how much power Tsvangirai has. The 27 EU ministers said in a joint statement they will watch for the agreement's implementation, especially the "immediate cessation of all forms of intimidation and violence." Tsvangirai and his party won most votes in March elections, but not enough to avoid a presidential runoff. An onslaught of state-sponsored violence against Tsvangirai's supporters led him to drop out of the runoff and Mugabe was declared the winner of a second vote widely denounced as a sham. The power-sharing deal addresses the thorny issue of land, with the parties accepting "the irreversibility" of the seizures of white-owned farms since 2000. They recognize that "colonial racist land ownership patterns ... were not only unsustainable, but against the national interest, equity and justice." They call for Britain, the former colonizer, to pay any compensation for seized land.